<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <script type = "text/javascript">
        var a = [1,2,3,"abc"];
        alert(a);
        var b = [];
        alert(b.length);
        b[0]= 1;
        alert(b.length);
        b[3] = "12"
        alert(b.length);
        for(var i = 0;i<b.length;i++)
            alert(b[i]);
    </script>
</body>
</html>